This disclosure generally relates to methods and apparatus for fabricating composite structures. In particular, this disclosure relates to methods and apparatus for curing composite structures, such as honeycomb sandwich composite structures.
Airplane manufacturers are under increasing pressure to produce lightweight, strong, and durable aircraft at the lowest cost for manufacture and life-cycle maintenance. An airplane must have sufficient structural strength to withstand stresses during flight, while being as light as possible to maximize the performance of the airplane. To address these concerns, aircraft manufacturers have increasingly used fiber-reinforced resin matrix composites.
These composites provide improved strength, fatigue resistance, stiffness, and strength-to-weight ratio by incorporating strong, stiff, carbon fibers into a softer, more ductile resin matrix. The resin matrix material transmits forces to the fibers and provides ductility and toughness, while the fibers carry most of the applied force. Unidirectional continuous fibers can produce anisotropic properties, while woven fabrics produce quasi-isotropic properties. Honeycomb core is often sandwiched between composite sheets to provide stiff panels having the highest specific strength. More specifically, honeycomb core sandwich panels or composite structures, which typically comprise composite laminate skins co-cured with adhesives to the honeycomb core, are widely used in aerospace applications, among others, because of their high stiffness-to-weight (i.e., “specific stiffness”) and strength-to-weight (i.e., “specific strength”) ratios.
Honeycomb sandwich composite structures may be fabricated utilizing various composite forming methods. The most commonly employed technique involves the use of a vacuum bag molding assembly wherein an impervious membrane or “vacuum bag” is employed for consolidating the composite skins or layers and ensuring proper adhesion thereof to the centrally disposed honeycomb core. More specifically, the lower or base composite skin, the honeycomb core, and the upper or face composite skin are sequentially laid in a rigid mold member so that the honeycomb core is overlaid or covered by the upper and lower composite skins. The upper and lower composite skins are typically formed from uncured “prepreg” or “B-stage” laminates that comprises a fiber reinforcement such as graphite, aramid, or fiberglass fibers (e.g., linear, weaves, or both) disposed in a binding polymeric matrix such as epoxy, phenolic, or other similar organic resinous material. Film adhesive typically forms the bonds between the upper and lower composite skins and the honeycomb core. A vacuum bag is disposed over the rigid mold member and seals thereto, thereby forming a mold cavity that is occupied by the uncured/unbonded composite lay-up. The mold cavity is then evacuated to subatmospheric pressure within the mold, and superatmospheric pressure is applied to the exterior (in an autoclave), and the temperature of the composite lay-up is increased while in the autoclave to cure the lay-up. The combination of subatmospheric internal pressure and superatmospheric external pressure consolidate the composite skins, remove air and volatiles from the resin binder, and apply the necessary compaction pressure to ensure full and uniform adhesion of the lay-up.
Because of the noise regulations governing commercial transport aircraft, high bypass engines incorporate acoustic panels within the nacelles. Conventionally, these elements are made with an inner perforated skin, a surrounding buried septum honeycomb core, and a non-perforated outer skin.
Curing of complex composite nacelle structures traditionally requires an autoclave to provide the temperature and pressure necessary for cure. Due to the high capital cost, autoclaves are typically sized to cure multiple parts in a batch, and the cost of purchase and operation is high due to the volume. Other methods of manufacturing, such as resin infusion, have been successfully used to eliminate the need for an autoclave or oven. However, nacelle honeycomb core composite structures, including nacelle acoustic structures in their current architecture, cannot be readily manufactured using other methods.
Improvements in methods for curing nacelle honeycomb core composite sandwich structures that reduce costs and increase production rates are wanted.